Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If it is over a year old, but kept in a tank with no fish, no other livestock that might be an alternate host (some fish parasites can also parasitize snails) then I would assume that the greatest majority of fish parasites and pathogens are dead. While many can enter some kind of resting or dormant phase, I doubt most would stay dormant for over a year.
I never try to save mulm, usually I just clean out the old substrate with whatever comes with it and rinse it reasonably well for storage. I do not bother saving mulm. On the other hand, I also do not go to great lengths to remove it or to sterilize the tank when I am changing things around like this. Net result: Whatever microorganisms may have been in the substrate probably got removed. Stuck to the glass: Still there, if they can tolerate drying out for a few hours to a few weeks. In the filter: Still there. I do try to keep the filters cycled if I know I am getting more fish, soon. So even if I have to run the filter on a 5 gallon bucket, I do that and add ammonia to keep the nitrifying bacteria alive. I assume all the other microorganisms are alive, too. For my tanks, then, the cycled filter is the major seed source for the new tank.